It seems unlikely that an event as momentous as the Reaper's arrival would be appropriate for DLC. Speculating seems rather useless, but its more fun than studying for midterms.
Arrival could refer to any number of things. The arrival of truth, war, the beginning of the end, etc. Truth is a possibility that seems interesting and feasible. Bioware has been heavily hinting that there is much more going on behind the scenes than we are privy to with Commander Shepard. The secrets of the Greybox, LotSB, comics, etc have all alluded to a connection between the Alliance, Cerberus, TIM, Shanxi artifact, and the Reapers.
Identifying possible enemies based on information we have available provides for the development of interesting theories.
We are very likely dealing with sentient antagonists (i.e. not Husks). These antagonist can assault in waves with in game mechanics, lend themselves to either a stealth mission or something of a similar nature, and can possibly be holding the doctor captive.
The primary reason I believe the antagonist will be sentient is that the events surrounding Object Roe specifically refer to a battle. The Battle for Object Roe, imho, lends itself to a large scale conflict that involves more than just mindless zombies.
Object Roe has to be something that can be fought for on foot. We are after all surviving five waves of enemies as we try to secure this objective. That precludes the Mass Relay some have suggested, unless you are on a ship... but why would there be boarding actions when we are destroying a Mass Relay.
The device the Turians brought to Shanxi appears to be the likely candidate. I believe it was mentioned in ME1 or somewhere else that the Reapers had indoctrinated thousands to act as sleeper agents. When they arrived, these agents put plans into action to further weaken resistance.
What if the Turian Hierarchy has been indoctrinated at a level higher than Saren. They wanted to use the device to indoctrinate human leadership, not realizing Shanxi was merely a colony. When they realized that Jack/TIM had apparently had greater interaction with the device than they originally presumed... well that could cause some serious problems, as the device acted more like a Prothean beacon than an indoctrination device. It would explain the assassination plot the Shadow Broker had for TIM (I don't remember this myself, I read it recently but I don't remember where... so I could be mistaken.)
What if your fighting elements of the Turian Hierarchy to.... never mind this is getting outrageous.
Its the Batarians... bunch of GOOUDAMMED TERRORIST!